Two-wire radio transmission



J. D. BOOTH ETAL Two-WIRE RADIO TRANsM-1ss1oN Filed Aug. 2, 1940 Feb. 2, 1943. 2,310,060

' 2 Sheets-Sheet 1 (i James Q 500th alla b, "9 w43. J. D. BOOTH ETAL TWO-WIRE RADIO TRANSMISSION Filed Aug. ,2, 1940 2 Sheets-Sheet 2 WITN ESSES: @J l l ATTORN Patente-d Feb. 2, 1943 Unir matt

TWO-WIRE RADIO TRANSMISSION James D. Booth and Ashley P. Bock, Catonsville,

Md., assignors to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 2, 1940, Serial No. 349,526

9 Claims.

This invention relates to communication systems, and more particularly to carrier frequency communication utilizing a single channel for simplex operation.

Systems of the above type may have several communication centers or stations each being equipped with a transmitter and a receiver. Since only a single communication channel is available, the transmitter, as well as the receiver, must operate on the same carrier frequency. 'I'his presents a serious problem of preventing interaction between the transmitter and the receiver located at the same station and at the same time each of these must be readily available for operation in quick succession where communication is carried out between two remotely located stations. In other words, when, let us say, at station A the operator is speaking to stay tion B, the transmitter at A must be functioning whereas the receiver at A must be inoperative. On the other hand, at station B the receiver must function and the transmitter must be inoperative. At the next instant, when the operator at B wishes to answer A, the order must be reversed and the receiver at 'A is to become operative, whereas the transmitter must be silenced, not only with respect to the modulation comprising the speech of the operator at station A, but also the carrier irequency, inasmuch as there can be no two carriers at the same time.

The operation of a. system of this type has various other ramiilcations which will be taken up in detail hereafter. Suiilce it to say that heretofore such systems required elaborate switching arrangements in order that communication could becarried on without much loss of time in the same manner as, for example, over a simple telephone line. 'Ihese switching arrangements employed a considerable number of relays and other mechanical transfer switches to interchange the transmitting and receiving cir- -cuits and effect energization of different elements of the circuits. Mechanical relays have the inherent disadvantageof possessing mechanical inertia. Furthermore, difficulties are caused by moving contacts in chattering and arcing which, in systems of the type herein considered, seriously interfere with smooth operation.

The major switching in prior systems involved the use of transfer switches which often had to be manually operated for going over from transmitter to receiver operation, and vice versa. In such systems both the input and output circuits ofthe transmitter and the receiver terminated at switch'contacts. The necessity of having these circuits alternately connected and broken proved to be a serious disadvantage. 'This condition was somewhat ameliorated by terminating the circuits in hybrid coils which needed careful and dimcult balancing of input and output networks.

'I'he presentinvention has for its primary object automatic simplex operation without any of the above mentioned disadvantages by providing electronic means for automatically conditioning the various circuits for operative or inoperative function as demanded by the normal use of the system.

A particular feature of this invention is that the system obviates the necessity oi using mechanical switching means in any form.

Another advantage offered by the communication system in accordance with this invention is that the hybrid coils for terminating the circuit are dispensed with and circuits heretofore so terminated can be directly interconnected.

A further object of primary importance of the system in accordance with this invention resides in the increased exchange rate of communication over a single channel by providing electronic control circuits having operating time constants less than the syllabic time of human speech.

Other objects and advantages will be apparent from the following description of the invention, pointed out in particularity by the appended claims, and taken in connection with the accompanying drawings in which:

Figure 1 is a block diagram showing the arrangement of several communication centers employing carrier current over a power line circuit, and

Fig. 2 is a schematic circuit arrangement of a transmitting and receiving system which may be employed at any one of the stations shown in Fig. l.

The invention finds particular usefulness in carrier current communication which employs power lines as the medium for conducting the carrier frequency current from one communication center to the other. In such systems single frequency operation is particularly advantageous in that it simplifies the equipment and makes full use of the limited facilities which a power line oiers for conducting carrier frequency current. For this reason the invention is stated and described as a carrier current system, although no limitation is intended thereby and there is no difference in the general basic organization of the circuits whether the transmitted high frequency energy is carried over wires or is emitted through-the ether space.

riihe power line is indicated by conductors I, 2 and 3 which may be a direct current line or twophase or three-phase alternating current. The carrier frequency is superimposed on one pair of lines shown here as 2 and 3 by means of a coupling system employing ccndensers 4, 5, 6 and l', 5', 6' in series to minimize the breakdown of the high voltage of the power line. These condensers offer a high impedance to the power frequency and a comparatively low impedance at carrier frequency. In series with condensers I, 5 and 8 and terminating to ground is a radio frequency reactance 1, and similarly in series with condensers 4', E' and 8', is a radio frequency reactance T', The latter are for the purpose of obtaining a comparatively high impedance to the carrier frequency to be transmitted. A line tuning unit is provided between the coupling circuit and the transmitter, shown here in block form, to match the line impedance with the output circuit of the transmitter. The transmitter and receiver are shown in one block diagram, since this invention permits permanent interconnection of these units as will be seen later, and both are permanently tied in with the line tuning unit. Blocks 'I' and T indicate telephone apparatus connected to the transmitter and receiver. The other units of Fig. 1, each representing a communication center, are identical with the one just described. Similar components thereof are marked with identical reference characters.

Referring to Fig. 2, the essential circuit elements of a complete transmitter and receiver are shown here in a simplified form. The invention refers to the novel electronic control circuits and may be applied to any type of receiver and transmitter. For the purpose of simplicity the transmitter circuit components include all the essential elements for a complete operative unit. Similarly, in the receiver a simple tuned radio frequency circuit was selected. There is no limitation intended in showing these circuits which can be modified as circumstances may demand to have more selectivity by including other tuned y circuits or by employing a superheterodyne type receiver. Similarly, in the transmitter other types of oscillators or modulators well known in the art may be employed to obtain any desired transmission characteristics.

Provision is made in accordance with this invention for the system to operate as follows:

l. When signals are being transmitted to render the receiver inoperative to full transmitter output Voltage prior to and during transmission of carrier.

2. Release the receiver block after carrier transmission has ceased yet soon enough to receive the beginning of the next part of the incoming conversation.

3. Rendering the transmitter inoperative by means of the received signal prior to its arrival at the receiving instrument and during the reception interval.

4. Release of transmitter blocking after reception has terminated but soon enough to prevent loss of any of the ensuing conversation.

Other features of the operation whereby the above requirements may be fulfilled are the time sequence characteristic of the control system and a level sequence providing an additional reduction in the time delayof action. This will be fully described in connection with the circuit shown in Fig. 2.

The components enclosed by one square in dotted lines represent the receiving apparatus, those in another square the signal frequency transmission circuit and part of the control circuit of the transmitter, Whereas those in the third square, the main components of the transmitter. A` simple telephone handset B provides for transmission and reception of signals and is connected by a telephone line comprising conductors 9 and IIJ in series with the customary microphone voltage source represented by the battery II to the input circuit of the transmitter and the output circuit of the receiver. These two circuits therefore are permanently tied together by the telephone lines 9 and I D. Similarly, the input circuit of the receiver and the output circuit of the transmitter are permanently tied together by conductors II and I 2 which ultimately terminate in the line tuning unit shown in Fig. 1. It may be mentioned here prior to discussing the circuit that a permanent interconnection of this type could not be effected heretofore because of the interaction that a transmitter would have on a receiver, and vice versa, resulting in a serious oscillation of the entire system referred to in telephone practice as singing A number of relays and switches had to be used to transfer the telephone lines 8 and I0 and the output-input conductors II, I2 to the circuits under operation.

The receiver includes in a conventional circuit arrangement the vacuum tubes I3, I4 and I5 in which the vacuum tube I3 performs the function of a carrier frequency amplier having its input circuit between grid I6 and cathode I1 connected to the conductors I I and I2 by a suitable coupling circuit. This includes the carrier frequency transformer I8 which is tuned by condenser I9 and a series tuned circuit comprising the primary coil 20 of the transformer I8, a series capacity 2| and the secondary winding 22 of the input transformer 23. The primary winding 2l of the latter connects directly Withthe conductors Il and I2. The cathode circuit between cathode I1 and ground includes a frequency responsive degenerative network comprising resistor 2'I shunted by capacity 26 and an inductance 21. The vacuum tube I3 is shown to be a pentode voltage amplifier deriving operating potentials from the source shown here by the battery 28.

Throughout the entire circuit arrangement shown in this figure, batteries were chosen to indicate the various voltage sources Wherei'rom static operating potentials are derived for the different tubes. In this manner the drawing and the explanation of the operation of the system can be greatly simplified. It is to be understood, of course, that no limitation is intended thereby, and in presentday practice, all these sources may be replaced by suitable rectified a1- ternating current power supplies. For the same reason the filament circuit of the tubes have been omitted for it is well understood that these tubes require heating currents winch may be derived from any suitable source.

In continuing with the description. the battery 28 has its negative terminal connected to the ground terminal side of the cathode I "I, and at an intermediate point suitable voltage is tapped oil for the screen grid electrode 29. The anode 38 is connected to the' positive terminal of the battery 28 in series with primary winding 3l of the which is tuned This forms the output circuit The secondary winding 34 of connected to a demodulator carrier frequency transformer 32 by condenser 33. of the tube I3. transformer 32 is v minal of the secondary winding 3B.

passed by the condenser 82. tap of the battery iili supplies the screen grid attacco comprising the duodiode tube it having anodes 35 and 36 which are interconnected with one ter- The latter is also tuned by condenser 31. The cathode 38 of this tube is connected to the junction point of resistors 39 and 39. The former is the load resistance of one diode section, its other terminal being connected to the low potential side of the secondary winding til of transformer 32 by conductor i0 and also to the control grid #3| of the vacuum tube i 5 through conductor 2. A condenser 43 connected between conductors Mi and 3e and in conjunction with resistor 35' serves as a modulation frequency lter for the input circuit of tube I5 as will be described in connection with the operation of the system. The vacuum tube I5 is essentially a direct current ampli er and Performs the function of the conventional automatic volume control tube in receiving circuits. The cathode "it thereof is connected t the common terminal of batteries 55 and 36. The latter provides a source of operating voltage supplyingthe screen grid t1 and the anode QB which has a. load resistance t@ by-passed by a conu denser 50, whereas the former supplies bias potential being eiectively connected between grid l and cathode M. The anode d8 connects also through conductor 5| to the cathode 52 of conn trol tube 53. This will be fully described in connection with the description of the control circuit. The other' portion of the duodiode tube i4, including cathode 511, may be referred to as the signal diode or detector of the system feeding the output amplifier tube 55. The output connection includes between cathode 54 and conductor d0 the load resistors 51 and 5B in series. The junction point thereof is connected to the grid 56 of tube 55 through a coupling network including resistor 59coupling capacity 60 and filter capacity 6| connected between the junction point of resistor 59 and condenser 60 and conductor di). Between the cathode t2 and the anode 33 of the output amplier tube 55 is the output circuit comprising the primary winding |54 of the output transformer, the secondary winding 65 of which connects to conductors 9 and I0, and the anode voltage source in the form of battery t6. Operating bias voltage for the input circuit of the tube 55 is obtained from the battery 61 through control tube load resistor 68 which is by-passed for signal frequencies by the condenser 69 and grid resistor' 10, the junction point of the two last-mentioned resistors being connected t0 the anode 1| of control tube 12.

The control circuit includes vacuum tubes 53 and 12, mentioned before, and vacuum tube 13. The anode circuit of the vacuum tube 12 was described in part comprising the anode 1| and load resistance 68. The operating source therefore is the other half of the battery 66, the negative terminal of which is connected to the cathode 1t which is also at ground potential.l The input to the control tube includes the control grid 15, resistor 16 and bias source battery 11, returning to the cathode 14. The grid 15 is also coupled directly to the output circuit of tube 13. This includes the resistor 18 and the anode 19 of the tube 13 in series with resistor 16, operating source battery 80, returning to the cathode 8|. Resistors 16 and 18 are thereby in series and by- A suitable voltage electrode 83 of tube 13. The input circuit of the control tube 13 includes the grid et, conductor Bt and also the secondary Winding 34 of the radio frequency transformer returning through con ductor di? and conductor @t to the bias source battery 81 and cathode tl. 'The anode 19 is also a point of interlinkage between receiver and transmitter, the conductor te being connected thereto terminating at the positive terminal of the bias supply battery' 9@ of the modulation fre-s quency amplifier of the transmitter.

Another interconnection is provided between receiver and transmitter by the conductor ii! which connects the cathode 92 of tube 53 and the anode of one of the control tubes of the transmitter which will be described later. The control tube 53 is a duodiode rectifier providing by its space current a path to the input circuit of the tube i3 for the automatic volume control derived from tube i5 and for control derived from the transmitter through the conductor 9|. The anodes 93 and 93 thereof are interconnected and terminate at the junction point of the grid return circuit of tube i3 and resistor t5 and by-'pass condenser 913 and bias source battery 9d which supplies the minimum operating bias for the grid i@ of tube i3.

The transmitter circuit comprises the followm ing conventional components. The iirst signal frequency amplifying stage including vacuum tube |05 followed by modulating stage having vacuum tubel ||i|,the oscillation generator including vacuum tube m2 and the nal output amplifier stage including vacuum tube |93. The other tubes represent the control circuit and will be described later.

Feeding the input amplifier is the coupling transformer |013, the primary winding W5 of which connects to the conductors d and i. The secondary winding it@ forms the input circuit of the tube '|68 between grid |111 and cathode |68 in series with the bias source battery Edt. Anode and screen grid potentials are derived from the battery im. The screen grid electrode l I l is by-passed by condenser H2. The anode circuit includes coupling transformer iid, the primary winding iid of which connects from the posit-ive terminal of the battery to the anode H5 of Ythe tube Hill. The secondary winding lit forms the input circuit to amplifying tube iti being connected to the grid i|1 thereof at one terminal, whereas the return terminal connects through the bias supply source battery @t to con=l ductor 88 which terminates at the anode 19 of the control tube 13. The output circuit of rnodu-n iator tube ii includes the anode i i8, the primary Winding ||9 of coupling transmitter ld, the anode potential source battery |2| and the cathode |22. The secondary winding |23 of the transformer |28 feeds into the primary winding |213 of the modulaton transformer |25. The secondary winding |26 thereof is in the input cirn cuit of the final amplifying tube ID3 comprising the grid |21', resistor |28, the secondary winding |26 and the grid bias battery |29 in series. The positive terminal thereof is connected to ground completing the circuit to the cathode itil of the tube |03 which also connects to ground. The output circuit of the amplifier tube |53 includes in series between cathode i3@ and anode 8.3i the plate potential source battery 132 and the primary winding 135 of the output transformer i3d. A suitable tap of the battery |32 connects to the screen grid electrode |35 which is by-passed by condenser E36. The secondary winding E31 of the output transformer i3d cgnnects to the conductors and i2.

The oscillator stage comprises a screen grid type vacuum tube |02 in a conventional feedback coupling. The cathode |38 thereof is connected to ground and the grid circuit includes grid electrode |39, grid load resistor |40 terminating to ground. The anode circuit includes the supply source battery |4|, radio frequency choke coil |42 and the anode |43 of the tube |02. The coupling between anode and grid circuit is eifected by the variable inductance |44, one terminal of which connects to the grid |39 and the other in series with condenser |45 to the anode |43. The oscillator circuit includes also series condenser |46 between grid |39 and ground and voltage distribution condensers |41 and |48 in series between ground and the high potential terminal of the variable lnductance |44. The junction point between condensers |41 and |48 is utilized in coupling the carrier frequency energy to the grid |21 of the final amplifying tube |03 through coupling capacity |49 and conductor |50.

The control circuit has two main branches, each operating on the rectified component of the signal frequency energy which is derived from two diode rectifiers |5| and |52 which are energized from the amplifier tube |53. One of the control tubes |54 operates from two energizing sources, one of which is the output of the diode |5|, and the other the output of the oscillator |02, whereas the other control tube |55 operates from the output of the diode |52.

Following the circuit, the amplifier tube |53 is energized simultaneously with amplifier tube |0| from the secondary winding ||6 of the coupling transformer ||3 in that the grid electrode |56 thereof connects to the grid ||1 of the tube |0|, and the cathode |51 to the cathode |22'. The source |2| is also utilized for anode potential through the primary winding |58 of the transformer |59 connecting to the anode |60. The transformer |59 has two secondary windings |6| and |62. arrangement connects at both terminals to the anodes |63 and |63 of the diode |5| and the midtap of this winding is returned to the cathode |64 through load resistance potentiometer |65.

cathode terminal. This output is fed to the grid |61 of the control. tube |54 in series with radio frequency choke coil |68 and resistor |69, the' cathode |10 being connected to the terminal of the potentiometer |65 through bias battery I 1|. Anode potential for the control tube |54 is obtained from the battery |12, the positive terminal of which is grounded and the negative connected to the cathode |10. Screen grid voltage is taken from a suitable tap to the screen grid electrode |13 by-passed by condenser |14. The anode |15 connects to the conductor 9| previously mentoned in connection with the description of the receiver. The anode circuit of the control tube |54 may be traced through the conductor 9|, the space current path of the diode 53. resistor 95, battery 96 to ground which is the positive terminal also of the battery |12.

The second diode |52 is similarly energized from the transformer |59, the secondary Winding |62 thereof being connected between anodes |16 and |16. The midtap in series with load resistance potentiometer |11 connects to the cathode |18. The loadresistance |11 is paralleled by condenser |19. The rider |80 of the potentiometer |11 connects to the control grid |8| of the control tube |55, returning to the cathode |62 thereof 'Ihe former in a. full wave rectifying- `in series with load resistance and the anode |86. Screen grid voltage is derived from a suitable tap which connects to the screen grid electrode |81. A portion of the battery voltage is utilized also to supply operating voltage to the screen grid electrode 98 of the oscillator tube |02. This is accomplished by grounding a suitable point of the battery |84 and tying the screen grid electrode 98 to the anode |86 through conductor |88. This completes the description of the circuit. The various phases of the operation of the receiver and transmitter as a complete unit shall now be described.

Every communication center of the system includes a complete assembly of receiver, transmitter and control circuit as shown in Fig. 2, forming thereby an operating unit in the chain supplied by the communication channel. Let us assume that it is desired to send a message from this station and the microphone of the handset 8 receives voice frequencies. The audio-frequency energy is fed through the transformer |04 and is ampliied by vacuum tube |00, and also further ampliiied by vacuum tube |0|, the output of which is delivered to transformer |20. This energy then modulates the carrier frequency .which is impressed on the grid |21 of the tube |03 through the condenser |49. The energy is then delivered by conductors |2 and which, as shown in Fig. 1, connect to the line tuning unit through which it is delivered to the power lines 2 and 3.

In addition to this function, audio-frequency energy is also delivered from the transformer I3 to the grid |56 of control amplifier tube |53. The output thereof is delivered to transformer |59 being rectified by both diodes |5| and |52. The output voltage of diode |5| appears across potentiometer |65 and is applied to the grid |61 of the tube |54. The latter is normally biased to plate current cutoff by the battery |1|. Simul- Y.,

taneously the output voltage of the diode |52 appears across potentiometer |11 and is applied to the grid |8| of control tube |55. The latter is biased by the battery |83 for normal plate cur. rent at no signal conditions. The voltage between cathode |82 and ground is so adjusted that it is greater than the voltage drop of the tube |55 through load resistance |85 thereby applying a slightly negative voltage to the screen grid 98 of the oscillator |02 with respect to its cathode |38 which prevents the oscillator tube from generating lcarrier frequency energy. However, as soon as signal reaches the transformer |59 the voltage appearing at |11 will be more negative and cuts off the anode current of the tube |55. Hence the potential between the anode |86 and ground becomes positive and the screen grid 98 will be normally energized, -permitting the tube |02 to oscillate and thereby produce carrier frequency energy.

Prior to discussing the function of the other control tube |54, reviewing the operation of control tube |55, itis seen that normally the oscillator tube |02 has negative potential on the screen electrode 98 and thereby is prevented from oscillation. The signal amplifier tubes, on the other hand, are in a stand-by condition, and as soon as signal frequency appears in the modulation circuit the negative bias is removed from the screen electrode and the oscillator becomes in- .stage, and will also tube |53 and diodes stantly operative. As soon as there is no audiofrequency signal, the oscillator again becomes inoperative. The time sequence of operation is governed by the constants of the control circuit, and it is important that these constants be of a predetermined character. This will be further explained as the operation of the 'circuit is unfolded.

Simultaneously with the energization of the oscillator to an operative state, the control tube |54 will receive bias voltage on its grid |61 from the output of the diode II. This tube is normally biased to plate current cutoif bythe battery I1I.V The control voltage produced by the diode I5I will oppose the voltage of the battery I1 I, permitting the now of anode current of theI tube |54 which ows from ground through battery 96, resistor 95, diode 53 and anode |15, returning to the negative side of the battery |12. The voltage produced by the current iiow across the resistor 95 applies suiiicient negative bias to the control grid I6 of tube I3 to render the input amplifier stage of the receiver inoperative to the full voltage of the output of the transmitter which will appear across the primary winding 24 due to the fact that it is connected also across conductors |I and I2.

The time required t block the receiver to inoperative condition is a function of the values of resistor 95, condenser 94 and the plate resistance of the tube |54. The time for actuating the carrier at the tube |03 is a function of the values of the capacity |19, resistor |11, capacity 99 and resistance of the screen grid 98 to the cathode |38. The values of the above-mentioned components are so proportioned that the receiver is blocked to inoperative condition prior to the beginning of generation of radio frequency energy by the tube |02.

Recapitulating briefly the operation, as soon as the microphone is energized, signal frequency currents will appear in the modulator amplier produce through the control |5I and |52 two sets of control voltages. One is utilized to trigger the oscillator tube into action, the latter being normally in a stage of inoperativeness, and the other voltage is utilized to cut of the receiver which is normally in a receptive state.

The receiving functions operate as follows. Upon impression of carrier frequency voltage on the input terminals of the primary winding 24 of transformer 23, the signal is amplied through the first stage including amplifier tube I3, and is delivered to the demodulator tube I4 and also through the conductor 85 to the grid 84 of the control tube 13. The signal from the demodulator tube is impressed on the grid 56 of the amplier tube 55 through the capacity from the load resistances 51 and 58 of the diode section comprising anode 35 and cathode 54. The other diode section comprising anode 36 and cathode 38 produces a unidirectional voltage which is fed through the resistor 39' by-passed by capacity 43 to the grid 4| of the automatic volume control tube I5. The operation of this tube is in the conventional manner in that the increased signal voltage produces an increase of positive voltage on the grid 4| causing plate current which is in such direction as to apply a negative voltage to the cathode 52 and thereby control the bias on the control grid I6 ofthe first amplifier tube I3. As statedbefore, the carrier frequency is also impressed on the grid 84 of tube 13.

olf the first amplifying stage l latter is normally biased to cutoff point by the battery 81. The impressed signal causes plate current flow from the anode potential battery 80 through the resistors 16 and 16. The other control tube 12 is biased by the battery 11 to normal plate current flow which, while it is maintained, is in such direction through'resistor 68 as to bias the output stage of the receiver, namely, the grid 56 of tube 55 to cutoff condition. Upon carrier frequency appearing on the grid 84 of tube 13, plate `current conductor is initiated Which, flowing through the resistor 16, will v deliver additional negative bias to tube 12 initiating cutoff condition, thereby removing the negative bias on the grid 56 caused by plate current through resistor 68. Thus, We have an inoperative receiver as to its final output stage until such time as a carrier signal either modulated or unmodulated is applied to the vacuum tube 13. The release of current through resistors 16 and 18 is also in such a direction and magnitude as to apply cutoi bias to vacuum tubes |0| and |53. The time required to cut on tubes IIlI and |53 is a function of the values of capacity 82, and the anode impedance of tube 13. The time required to release the cutoff bias from vacuum tube 55 in the output stage of the receiver for delivery of signal to the telephone receiver 8 is a function of the values of resistor 68 and capacity 69. The values of the above-mentioned components are such that the vacuum tubes IIlI and |53 are cut oi to non-operation prior to the removal of cutoff bias holding the tube 55 produced by current flow through the resistor 68.

It is seen from the above that the receiver is blocked to inoperative condition prior to and during the generation of radio frequency energy from the transmitter. The latter condition is assured by the coupling of the oscillator grid |39 to the grid |61 of control tube |54 by means of the capacity 91 which connects to the junction point of the resistor |69 and choke coil |68, thereby some of the carrier is fed to the control tube |54 in order that the receiver shall be blocked as long as carrier is generated by the transmitter. Upon cessation of signal frequency, that is, audio-frequency the carrier stops after which the receiver is restored to operative condition upon the discharge of capacitor 94 through resistor 95. The receiver now is in a stand-by condition, and upon impression of a received signal the actuation of carrier by the transmitter is prevented by blocking oi the tubes and |53 prior to the delivery of the received signal to the telephone 8 from the receiver. Upon cessation of received carrier the tube 55 will be blocked to inoperative condition by the tube 12 prior to or simultaneously with the restoration of normal operating bias to the tubes 'IIlI and |53.

In addition to the timing sequence described above, the principle of level sequence is also applied for the purpose of improving operation characteristics. For example, the signal frequency gain from the telephone to the grid |61 of tube |54 is great enough to block the receiver to inoperative condition at a level equal to or below that required to actuate carrier. Furthermore, tubes I6I and |53 are blocked to an inoperative condition at a signal level equal to or below that required to actuate the final amplier stage of the receiver that is, tube 55. If the precautions of timing and sequence are applied as described above, it becomes possible to obtainA automatic operation controlled entirely by the The normal conversational speech, that is, at less than syllabic time. A loss of time from the initiation of a speech tone at the input of the transmitter and the delivery of this tone at the output of the distant receiver can be made less than ten mini-seconds. This value has been found by experiment to be entirely negligible in its adverse effects to the utility of the system.

We claim as our invention:

1. In a communication system, a single communicating channel, a plurality of communication centers adapted to utilize said channel, each of said centers being equipped with means for transmitting and receiving signals, said means for transmitting embodying an oscillation generator, and control means responsive to the presence of signals in said receiving means for preventing said generator from producing any substantial output current.

2. In a communication system, a single carrier frequency communicating channel, a plurality of communication centers utilizing said channel, a transmitter at each of said centers including a carrier'frequency generator, a signal frequency amplifier and a carrier frequency modulator, a

receiver at each of said centers including a carrier frequency amplifier, a demodulator and a signal frequency amplifier; control means interconnecting said transmitter and said receiver,

said means being selectively responsive to carrier frequency in said channel and signal frequency in said transmitter for reducing the output current of said carrier frequency generator substanmeans for causing energization of said receiver to block said modulation frequency input ampliner.

.5. The arrangement dened in claim 4 provided With timing means to insure that said modulation frequency input amplifier is blocked prior to the unblocking of said modulation frequency output amplifier.

6. In a communication system, a station comprising a transmitter and a receiver, said transmitter embodying an oscillation generator and an audio frequency input circuit therefor, means responsive to current flow in said audio frequency input circuit for blocking said receiver, and additional means responsive to alternating voltage on said oscillationgenerator for blocking said receiver.

'7. In a communication system, a station comprising a transmitter and a receiver, said receiver embodying a tube having a control electrode, means for supplying to the circuit of said control electrode a first current governed by the strength of an incoming signal to said receiver, means for supplying to the circuit of said control electially to zero and simultaneously making said signal frequency amplifier of said receiver operative upon the presence of carrier frequency in said channel, and means for rendering the carrier frequency amplifier of said receiver inoperative and the carrier frequency generator of said transmitter operative upon signal frequency in said signal frequency amplifier of said transmitter.

3. In a communication system,` a station com.. prising a transmitter and a receiver, said receiver including a carrier frequency input amplifler and a modulation frequency output arnplier, means for blocking said modulation frequency output amplier when no signal is being l jre'c'eived, and means responsive to the signal output of said carrier frequency input amplifier to unblock said modulation frequency output amplier.

4. In a communication system, a station comprising a transmitter embodying a modulation frequency input amplifier and a receiver, said receiver including a carrier frequency input amplifier and a modulation frequency output amplier, means for blocking said modulation frequency output amplifier when no signal is being received, means for causing energization of said carrier frequency input amplifier to unblock said modulation frequency output amplifier, andl trode a second current governed by the energy input to said transmitter, a first diode traversed by said first current, a second diode traversed by said second current, one set of corresponding electrodes of said diodes being connected together to one terminal of a resistor, and two sources of voltage being respectively connected in circuit between the other electrodes of said two diodes.

and the other terminal of said resistor.

8. A communication system in accordance with claim 2 wherein said means for rendering the carrier frequency amplifier of said receiver inoperative comprises means for producing a control potential in response to energy in said signal frequency amplifier, and said means for rendering said generator operative comprises similar means distinct from said first-mentioned means for producing a control potential in response to energy in said signal frequency amplifier.

9. A communication system in accordance with claim 2 wherein said means for rendering the carrier frequency amplifier of said receiver inoperative comprises circuit elements for producing a control potential in response to energy in said signal frequency amplifier, and said means 'for rendering said generator operative comprises similar circuit elements distinct from said first- 

